In recent years, in an optical pickup device, a wavelength of a laser light source which is employed as a light source for reproducing information recorded on an optical disc and for recording information on an optical disc, is becoming shorter. For example, laser light sources having wavelength of 390-420 nm, such as a blue-violet semiconductor laser, are reaching the stage of practical application. A use of such the blue-violet laser light sources enables to record information of 15-20 GB on an optical disc with a diameter of 12 cm, when an objective lens with a numerical aperture (NA) which is the same as that for a DVD (Digital Versatile Disc) is used. When the NA of the objective optical element is increased up to 0.85, information of 23-25 GB can be recorded onto an optical disc with a diameter of 12 cm.
As an example of an optical disc using the above objective lens with NA of 0.85, there is cited a BD (BLU-RAY DISC). In a BD, since coma which is generated because of an inclination (skew) of an optical disc increases, a protective substrate is designed to be thinner than that of a DVD (to be 0.1 mm, while it is 0.6 mm for a DVD), to reduce the amount of coma coming from a skew.
Besides, it is hard to say that a value of an optical disc player/recorder (optical information recording and reproducing device) as a product is not enough under just a condition that it can record/reproduce information for BDs properly. Taking account of a fact that DVDs and CDs (Compact Discs) onto which various kinds of information have been recorded, are on the market at present, it is not enough that the device can record/reproduce information only for BDs properly. For example, realization of recording and/or reproducing information appropriately also for DVDs and CDs which are owned by users, leads to enhancement of its commercial value as an optical disc player/recorder for BDs. From such the background, an optical pickup device installed in an optical disc player/recorder for BDs is required to have a function capable of appropriately recording and/or reproducing information not only for BDs but also for DVDs and CDs.
As a method capable to record and/or reproduce information adequately to anyone of BDs and DVDs and further to CDs, with maintaining compatibility, there can be considered a method to selectively switch an optical system for BDs and an optical system for DVDs and CDs, corresponding to a distinguishing feature of a BD, DVD or CD on which information is recorded and/or reproduced. However, it is disadvantageous for the size-reduction and increases the cost, because it requires plural optical systems.
Accordingly, in order to simplify the structure of an optical pickup device and to intend the reduction of cost, it is preferable to make an optical system for BDs and an optical system for DVDs and CDs into a common optical system, and to reduce the number of optical parts constructing the optical pickup device as much as possible, in the optical pickup device with the compatibility. Then, providing the common objective lens which is arranged with facing an optical disc, is most advantageous for the simplification of the construction and cost reduction of the optical pickup device. In order to obtain an objective lens which can be commonly used for plural kinds of optical discs for which different recording/reproducing wavelengths are used, it is required that an optical path difference providing structure having a wavelength dependency in a spherical aberration, is formed on the objective lens.
Patent Literature 1 discloses an objective lens which includes a structure wherein two basic structures each being a diffractive structure are overlapped together and can be used commonly for three types of optical discs, and discloses an optical pickup device equipped with the objective lens.
As one of great problems of an objective lens which can be commonly used for three types of optical discs, especially an objective lens which can be commonly used for three types of optical discs of a BD, a DVD and a CD, there is a problem combing e securing the axial thickness which is suitable for handling a BD with securing a working distance which is suitable for handling a CD having a thick protective substrate. Further, it is required to realize a stable mass-production and an easy installation of the lens to an optical pickup device while achieving the solution of the above problem.
However, Patent Literature 1 does not disclose concrete examples of a ratio of a diameter of an effective aperture on an optical surface and concrete examples of a flange shape. Patent Literature 1 does not disclose which kind of shape is preferable as the overall structure of an objective lens, other than optical surfaces, from the viewpoint of the stable mass-production and easy installation to an optical pickup device, under the condition that both of securing the axial thickness suitable for handling a BD and securing a working distance suitable for handling a CD having a thick protective substrate have been achieved.